Photoelastic material has the optical properties of polarizing light when under stress and transmitting such light on the principal stress planes. The velocities of the light depend on the strain in the workpiece, which phenomenon is known as "birefringence." The photoelastic material is clear, elastic and should be homogeneous, optically isotropic when under no stress or strain, and reasonably free from creep, aging, and edge disturbance. Examples of photoelastic material are Bakelite, celluloid, gelatin, synthetic resins, glass and other such commercial products that are optically sensitive to stress and strain.
When the photoelastic material is subject to monochromatic polarized light, the birefringence of the photoelastic material causes the light to emerge refracted in two orthogonal planes. Because the velocities of light propagation are different in each direction, there occurs a phase shifting of the light waves. When the waves are recombined with polarizing film, regions of stress where the wave phase is canceled appear black, and regions of stress where the wave phase is combined appear light. When white light is used in place of monochromatic light, the relative retardation of the photoelastic material causes the fringes to appear in colors of the spectrum.
In using a photoelastic strain gauge to measure strain in the workpiece, the gauge is bonded to the workpiece surface and illuminated by polarized light. A photograph is taken of the gauge which depicts patterns that an experienced technician can "read," to determine the difference of the strain in the workpiece along the two principal directions. The principal directions of the strain have, in the past, been determined by noting the orientation of the polarizing device. Additional photographs are taken for additional orientations of the polarizing device, thereby to obtain the principal directions of strain throughout the area of the workpiece covered by the gauge. This procedure is too timeconsuming and expensive because of the need to change the orientations of the polarizing device and to take multiple photographs.
Also, in the past, a gauge was specially constructed to conform to the shape of the workpiece.